impl(bigtable): make dynamic pool options available and update integration tests

[scotthart]

This PR enables instance bound connections with dynamic channel pools.

fixes #16035

[gemini-code-assist]

Summary of Changes

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This pull request refactors the Bigtable client library to expose dynamic channel pool configuration options to users. By moving the InstanceChannelAffinityOption and DynamicChannelPoolSizingPolicy to the public options.h header, it enables developers to configure instance-bound connections with dynamic channel pools. The changes also include updates to internal implementations and integration tests to support and validate this new configurability, allowing for more flexible connection management based on workload patterns.

Highlights

• Public API Exposure: Moved InstanceChannelAffinityOption and DynamicChannelPoolSizingPolicy from internal headers to google/cloud/bigtable/options.h within the experimental namespace, making these dynamic channel pool configuration options publicly available.
• Internal Code Updates: Updated internal implementations, including DynamicChannelPool and related tests, to reference the newly exposed experimental::DynamicChannelPoolSizingPolicy.
• Enhanced Integration Testing: Modified integration tests to allow conditional enabling of dynamic channel pools via an environment variable, and updated Bazel build configurations to run tests with both static and dynamic channel pool settings.

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[gemini-code-assist]

Code Review

This pull request refactors the InstanceChannelAffinityOption and DynamicChannelPoolSizingPolicy by moving their definitions from internal headers to google/cloud/bigtable/options.h within the experimental namespace, making these experimental channel pooling configurations publicly available. The changes also update internal code to reflect the new namespace and enhance the testing framework by adding variations to BUILD.bazel and modifying table_integration_test.cc to enable and test the dynamic channel pool feature via an environment variable. A critical issue was identified where the pool_size_increase_cooldown_interval member was inadvertently omitted from the DynamicChannelPoolSizingPolicy struct in options.h, which needs to be re-added to prevent compilation errors and ensure correct behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.69%. Comparing base (d60f99e) to head (bb3facd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16056      +/-   ##
==========================================
+ Coverage   92.68%   92.69%   +0.01%     
==========================================
  Files        2342     2343       +1     
  Lines      216537   216541       +4     
==========================================
+ Hits       200700   200727      +27     
+ Misses      15837    15814      -23     

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